1. Field of the Invention
The invention relates to a pulse-modulated source with adjustable parameters and to its use in an IFF or secondary radar emitting assembly.
In the field of IFF (Identification Friend or Foe) or secondary radar, different waveforms and modulations are used depending on the mode of operation of the IFF emitter as can be seen in the following table.
IFF emitting modeModulation usedMk X and Mk XII Interrogations &PulseResponsesMode S InterrogationsPulse + DPSKMode S ResponsesPPM type pulsesMode 5 Interrogations & Responses 5Pulses + MSK
Each IFF emitting mode has an associated modulation source placed in the emitting assembly. Thus, the IFF emitting assembly comprises four different sources for each of the four IFF emitting modes indicated here above. The source consists of a voltage-controlled oscillator VCO and a phase-locked loop PLL which gives a signal directly at the output frequency. The output of the source is then modulated to obtain the desired IFF signal.
A classic IFF emitting assembly therefore comprises:                a first source connected to two channels, each comprising an amplitude modulator to carry out the pulse modulation, the two channels corresponding to the two frequencies necessary, one for emitting and the other for testing the reception system in the Mk X and Mk XII interrogation and response modes,        a second source connected to two channels, each comprising an amplitude modulator and a DPSK modulator carry out the phase shift keying pulse modulation, the two channels corresponding to the two frequencies necessary, one for emitting and the other for testing the reception assembly in the Mode S Interrogation mode,        a third source connected to two channels, each comprising an amplitude modulator and an MSK modulator to carry out the minimum phase shift keying pulse modulation, the two channels corresponding to the two frequencies necessary, one for emitting and the other for testing the reception channel in the Mode 5 Interrogation and Response mode.        
The drawbacks of such an IFF emitting assembly are its complexity of implementation owing to the number of frequency sources and modulators used.
2. Description of the Prior Art
The U.S. Pat. No. 5,867,535 proposes to reduce the complexity of an emitting assembly necessitating the implementation of numerous radio functions by using a DSP (digital signal processor). From the information to be sent, the DSP generates the phase and quadrature signals and sends them to an FPGA (Field Programmable Gate Array). The FPGA then controls a modulator carrying out the modulation. The modulator then produces a modulated signal on an intermediate frequency comprising the information.
This solution, applied to the IFF emitters, would always call for at least two frequency sources and modulators in order to generate the DPSK and MSK frequency hops.
This is why, the number of IFF emitting modes used by the prior art emitting assembly is often very small as compared with the number of modes (24 different modes) provided for under the framework of IFF assemblies.
IFF type equipment is integrated into assemblies that are becoming increasingly compact. However, the architecture of currently used IFF emitting assemblies is such that there is a limit to the possible reduction in the volume occupied by such equipment, and this limit is soon reached.
Furthermore, the increase in the number of sources and modulators means that the precision of the IFF emitting assemblies in frequency is coarse in relation to manufacturing costs that remain high.
The present invention mitigates these drawbacks by eliminating the requirement that the emitting assembly should have a frequency source proper to each IFF emitting mode through the use of a digital frequency synthesizer capable of generating pulse trains.